1. Field of the Invention (Technical Field):
The present invention relates to the use of akaganeite-coated carrier media for the adsorptive removal of arsenic from water.
2. Description of Related Art
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-à-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
Arsenic contamination in surface water and groundwater systems is a result of both natural occurrences and human activities. Recent amendments to the United States Safe Drinking Water Act reduce the maximum contaminant level (the “MCL”) for arsenic from 50 μg/L to 10 μg/L. The economic impact of the new regulatory constraints on small communities is expected to be severe because traditional treatment technologies are not economically and technically feasible for use in the smaller systems used by such communities.
Depending on the source and area of the natural water, arsenic exists in a number of forms including different oxidation states, organic-bound, or either dissolved or particulate. The amount and form of arsenic in natural water depends on factors such as pH, particulate and natural organic matter concentrations, biological activity, and dissolved oxygen levels (Gregor, J. Water Resources 2001, 35, 1659).
Although a total of four oxidation states exist for arsenic, arsenic exhibits the two most common oxidation states when in an aqueous form, arsenite (also known as arsenic (III)) and arsenate (also known as arsenic (V)) (Sanchez, C. M. Batch Studies for the Treatment of Arsenic Contaminated Water Using an Iron Oxyhydroxide 2002, MS Thesis, New Mexico State University). In most natural water systems, the inorganic arsenic species are dominant—arsenate is more common in surface waters and arsenite is more common in ground waters. Over the typical pH range found in water treatment, arsenite exits as the uncharged H3AsO3 (Thirunavukkarasu, O. S. et al. Water Quality Research Journal of Canada 2001, 36, 55), and arsenate is typically present as HAsO42− (Vrijenhoek, E. M., Waypa, J. J. Desalination 2000, 130, 265). The organic forms of arsenic, MMAA and DMAA, are reportedly rarely present at concentrations above 1 μg/L (Thirunavukkarasu, O. S. et al. 2001). Arsenic speciation is also dependent on the source of water. Up to 60% of arsenic in surface water is reportedly organic, whereas organic arsenic is undetectable in groundwater using conventional measurement methods (Miller, G. P. et al., Water Resources 2001, 34, 1397). Geographical location also affects the occurrence of arsenic in water systems. A large number of geographic areas are of concern because the groundwater systems in those locations will be in violation of the new MCL of 10 μg/L.
Arsenic is an acute and chronic toxin and human carcinogen (Miller et al., 2000). When setting drinking water standards, the primary consideration is the carcinogenic effect of low level chronic exposure to arsenic (Pontius, F. W. et al. J. American Water Works Assoc. 1994, 86, 52). However, the health effects of the different species of arsenic vary. Arsenite is the most toxic, followed by arsenate, and the organic forms of DMA and MMA are least toxic (Pontius et. al., 1994).
Various approaches exist for the treatment of water contaminated with arsenic. The more common technologies can be classified as chemical coagulation/precipitation processes, membrane separation processes, and adsorptive processes. These processes were developed to target other contaminants in water treatment, but have been modified to improve the removal of arsenic.
Chemical coagulation, using either an aluminum-based or iron-based coagulant, is the most common treatment for arsenic removal. Coagulation combined with filtration has traditionally been used in surface water treatment to remove particulates and dissolved colloids in the water. However, removal by coagulation is difficult for dissolved species of arsenic. It has been reported that soluble arsenate is removable by adsorption to aluminum-based floc, but soluble arsenite is not removable by such a method (Gregor, 2001). Therefore, to effectively remove arsenite, it must be oxidized to arsenate. This is usually accomplished by pre-chlorination, which can lead to undesirable disinfection bi-products.
Several chemicals, including iron and aluminum, can be utilized in coagulation or precipitation processes. Such processes include lime softening, iron/manganese oxidation (US EPA Technologies and Costs for Removal of Arsenic in Drinking Water 2000, EPA-815-R-00-028), sulfide or hydroxide precipitation (Torrens K. D. Pollution Engineering 1999, 31, 25), and precipitation with lanthanum salt (Tokunaga, S. et al. Environment Research 1999, 71, 299).
Membrane separation is achieved through a selective barrier. However, a driving force or difference in potential is required to create movement through the membrane. The driving force can be pressure, concentration, electrical potential, or temperature (US EPA, 2000). Because of the system requirements, membrane separation can be costly. The most common type of membrane process is pressure driven. Pressure driven membrane systems are classified by pore size and include reverse osmosis (“RO”), nanofiltration, ultrafiltration, and microfiltration (US EPA, 2000). For membranes with smaller pores, more pressure is required, and that causes an increase in energy and costs.
For the removal of arsenic from groundwater, which contains 80% to 90% dissolved arsenic species, RO is effective (US EPA, 2000). During RO, feed water passes through the membrane by a pressure gradient. As the osmotic pressure increases, salts are rejected across the membrane and a concentrated discharge stream is produced. As in other processes, RO removes arsenate more efficiently than it removes arsenite. Therefore, an oxidation step may be added to the process for removing arsenite (US EPA, 2000). Although RO has reportedly removed up to 99% of total arsenic in source waters (US EPA, 2000), the system has disadvantages, which include higher volume, disposal of the concentrated discharge stream, and high operation costs.
Nanofiltration also has been researched as a lower cost alternative because of the large molecular weight of the typical arsenic species of H3AsO3 (126 g/mole) and HAsO42− (140 g/mole) (Vrijenhoek and Waypa, 2000). Electric potential membranes such as electrodialysis reversal (“EDR”) are efficient for arsenic remediation.
Adsorptive processes to remove arsenic are usually performed with activated alumina (“AA”). As in ion exchange, packed beds are used to remove contaminates from water by performing an ion exchange. Ions such as fluoride, arsenic, selenium, and silica are exchanged with the surface hydroxides on the alumina (US EPA, 2000). However, when all the adsorption sites are filled, the media must be regenerated. Parameters such as pH, oxidation state of arsenic, competing ions, empty bed contact time, and regeneration have a significant effect on the amount of arsenic removed (US EPA, 2000). Disadvantages of such a system include the disposal of the regenerants and spent media, and the effects on secondary water quality.
Akaganeite, a microscopic iron oxyhydroxide, is a very effective adsorbent for the removal of arsenate from drinking water. However, its application is limited to large systems where conventional flocculation followed by sedimentation may be used to remove the saturated adsorbent from water. For example, arsenic removal studies using akaganeite as a treatment have been developed by the Technical University of Berlin (Germany). In the University's studies, akaganeite is referred to as granulated ferric hydroxide (GFH), but is identified by the molecular formula β-FeOOH. The GFH is media of irregular grain size up to 2 mm in size, and reportedly contains 52-57% akaganeite (Jekel, M., Seith, R. Proceedings of IWA World Water Congress, Special Subject No. 13 1999, 16). Even though the GFH is an adsorbent, it operates as a particle filter, so turbidity can also be alleviated (Jekel and Seith, 1999). Therefore, the process is effective for particulate and soluble forms of arsenic. Operation parameters, such as pH and drying factors, affect the removal of arsenic. The GFH must not be dried, and the binding capacity decreases with rising pH (Jekel and Seith, 1999). GFH can treat between 70,000 and 80,000 bed-volumes, and could be implemented as a point-of-use treatment (Jekel and Seith, 1999).
Iron hydroxide laden zeolites (Canning, K. Pollution Engineering 2000, 32, 10) and the iron (II) laden zeolite minerals described in U.S. Pat. No. 6,042,731 have demonstrated an increased affinity for arsenic. Neither approach utilizes akaganeite.